Technico-economic analyses of the viability of fuel cells of the PEMFC (Proton Exchange Membrane Fuel Cell) type show that the future of this technology will essentially be determined by the reduction of the cost per kW supplied by the cell and the service life of its components.
One of the parameters directly affecting the cost of the cell components stems from the catalyst. Most effective catalysts for low temperature cells are noble metals, such as platinum (Pt) or ruthenium (Ru), which are very costly.
Thus, it has been estimated that the cost of the catalysts in the PEMFC fuel cell accounts for 70 to 80% of the total value of the core of the cell.
The prior art, for the fabrication of electrode materials, recommends deposition by spray-coating, painting or coating of an ink consisting of metal catalyst particles supported on particles of carbon and of an organic solvent. Such electrodes are described for example in documents EP-A-0 872 906 and EP-A-0 928 036.
The use of catalyst particles immobilised on carbon serves to obtain nanometer sized catalyst particles while reducing the catalyst load. However, the method for fabricating particles supported by carbon generally requires high temperatures (T>300° C.), which cause the coalescence of the metal particles. Moreover, catalysts supported on carbon have the drawback of sometimes being lost in the nanometer sized pores of the carbon, making them inactive in the active layer of the electrode. The content of platinum deposited, constituting the optimised active layer, is about 0.35 mg/cm2. This value is too high in terms of cost, and commercial analyses recommend platinum contents in the cell close to a value lower than 0.1 mg/cm2.
Furthermore, considering the active layers using C/Pt, it is estimated that only 50 to 75% of the fixed platinum is electroactive. The loss of platinum activity is essentially due to fractures on the electron or proton conduction networks, the input of reactive species in the electrode material (gas diffusion network) and the inactivity of the catalyst particles lost in the pores of the carbon.
Fabrication methods by electrodeposition by CCVD (described in document WO 03/015199) or by ion-beam (described in document U.S. Pat. No. 6,673,127) have revealed the possibility of directly immobilising the catalyst on the surface of the materials of the diffusion layer to overcome the drawbacks of carbon-supported platinum spray deposits. However, these technologies produce unsatisfactory active layers, for example due to the excessive size of the particles or the insufficient penetration of these particles into the diffusion layer.
An obvious need therefore exists to produce catalyst layers for electrochemical reactors, and particularly for PEMFC fuel cells, serving to avoid immobilising high catalyst loads, which are liable to remain inaccessible to the proton conduction and gas diffusion network, or to block electron conduction.